Thus 1° amines take up three molecules, 2° amines two molecules and 3° amines only one molecule of alkyl halide to form quaternary ammonium salts. Hence this reaction may be used for distinguishing three types of amines.
Reaction of amines with excess of alkyl halide to form a quaternary ammonium salt is known as exhaustive alkylation. Exhaustive methylation of amines followed by treatment with moist silver oxide and subsequent heating to form alkene is known as Hofmann elimination. For example,
\(\underset{n-Propyl\,amine}{CH_3CH_2CH_2NH_2}\xrightarrow{3CH_3I}\)
\(\underset{Trimethyl-n-propyl\,ammonium\,iodine}{CH_3CH_2CH_2N^ +(CH_3)_3I^ -}\xrightarrow{AgoH}\)
\(CH_3CH_2CH_2\overset{+}{N}(CH_3)_3OH^ - \xrightarrow{heat} \)
\(CH_3CH=CH_2+(CH_3)_3N+H_2O\)
This reaction is widely used in removing nitrogen from heterocyclic rings for elucidating the Structure of alkaloids under the name of Hofmann exhaustive methylation.
Acylation, benzoylation and alkylation are examples of nucleophilic substitution reactions.
